In recent years, high quality displays, for example, a full color display for a note book computer or for a mobile telephone, and a high resolution display, have been developed. These high quality displays are required to have a highly functional optical film exhibiting antireflective or antistatic properties. The surface of such a display is often touched so that the display is required to be resistant to scratching. Accordingly, a hard coat film is usually provided on the surface of a display. Recently, in order to improve visibility, displays are required to have antireflective and antistain properties. For these purposes, an optical film of a cellulose ester having a functional film (such as a metal oxide film) has been developed. The functional film may be provided directly or after forming an interlayer on the cellulose ester film. Also, a thinner hard coat film for a thinner display, or a wider hard coat film for a larger display is demanded. Specifically, for a large area display, a sufficiently flat hard coat film is desired. However, so far, a wide and thin hard coat film with superior flatness has been difficult to obtain. Also, a wide film with sufficient scratch resistance has yet to be obtained.
Generally, a cellulose ester film contains such an additive as a plasticizer and ultraviolet absorber. Such an additive is utilized to improve the processability and moisture permeability of the cellulose ester film. However, use of such an additive involves a problem of coating degeneration when the cellulose ester film is coated with various types pf coating layers. Bleed-out or volatilization of such an additive has raised the problem of causing a change in physical properties and degenerating the storage stability of the unprocessed cellulose ester film.
Further, the cellulose ester film absorbs moisture to expand and shrink. When the end of a wide roll film in particular is provided with knurling, the film is elongated or slackened by moisture absorption and expansion. This causes a roughened surface, streak and fold to be produced on the film. This problem has been particularly serious in a thin film. There has been a intense demand for improvement.
One of the solutions to the aforementioned problems is proposed in a laminated cellulose ester film based on the co-casting method for providing a distribution in widthwise direction of the film in the process of containing a plasticizer and ultraviolet absorber (Patent Documents 1 through 4). Another solution is found in the method of changing the type of the plasticizer or using a plurality of the types of plasticizers (Patent Documents 5 through 8). However, the advantages of these methods are restricted in a wide and thin film. Satisfactory improvement cannot be achieved in dimensional stability and flatness.
On the other hand, a method of including a polyester or polyester ether for improvement of dimensional stability has been disclosed (Patent Documents 9 and 10). It has been made clear, however, the cellulose ester film disclosed in the (Patent Documents 9 and 10) has a problem of sticking during a long-term storage. It has been revealed that, especially when the film is wound up in the form of a roll and is exposed to a high temperature and humidity, sticking tends to occur. This problem was not very serious in the earlier stage. However, when the film is transported to overseas countries by ship, the film is exposed to severe conditions of a high temperature and humidity for a long time. This problem has come to require a solution. Further, the winding shape of a film may be subjected to concave deformation during the long-term transportation. This problem also requires a solution.
To control the optical characteristics and physical properties, drawing must be made at a higher draw rate. To improve productivity, film production speed must be increased. This has resulted in a film more vulnerable to rupture. The solution to this problem is urgently required.
[Patent Document 1] Official Gazette of Japanese Patent Tokkaihei 8-207210
[Patent Document 2] Official Gazette of Japanese Patent Tokkai 2001-131301
[Patent Document 3] Official Gazette of Japanese Patent Tokkai 2001-151902
[Patent Document 4] Official Gazette of Japanese Patent Tokkai 2002-90544
[Patent Document 5] Official Gazette of Japanese Patent Tokkaihei 5-5047
[Patent Document 6] Official Gazette of Japanese Patent Tokkaihei 11-124445
[Patent Document 7] Official Gazette of Japanese Patent Tokkaihei 11-246704
[Patent Document 8] Official Gazette of Japanese Patent Tokkai 2001-247717
[Patent Document 9] Official Gazette of Japanese Patent Tokkai 2002-22956
[Patent Document 10] Official Gazette of Japanese Patent Tokkai 2002-120244